This invention relates to recording data in at least one longitudinal track on a magnetic tape data storage medium, the data having demarcation signals, and, more particularly, to providing a directory for the recorded data.
Magnetic tape provides a means for physically storing data which may be archived or which may be stored in storage shelves of automated data storage libraries and accessed when required. When a magnetic tape is accessed, not all of the data stored on the magnetic tape is necessarily accessed, and not all of the data is necessarily accessed in sequence. Hence, a directory is often provided to identify and locate the data stored in the magnetic tape data storage medium. An example of a directory structure is discussed in U.S. Pat. No. 5,613,082, in which displacements of the magnetic tape measured by a tape reel tachometer are used to reference physical locations of data blocks. Once the displacement of a desired data block is identified from the directory, a high speed locate movement may be made to the vicinity of the desired data. Another example is a xe2x80x9cfilemark tablexe2x80x9d which indicates the location of a filemark. A xe2x80x9cfilemarkxe2x80x9d is a specialized recorded signal that is used at the end of a data file, and thus comprises a signal indicating demarcation between data files. Access from one data file to another may be made by a space command from the present filemark, plus or minus a defined number of filemarks.
In one example, the magnetic tape is provided in a removable data storage cartridge together with a cartridge memory, and the directory and/or filemark table is stored in the cartridge memory. In another example, the directory and/or filemark table is stored in a specified location on the magnetic tape.
One difficulty is that the directory and the filemark table are open ended. Hence, in the case of a large number of data files, the capacity of the memory could be exceeded, or the back and forth traversing of the magnetic tape to access the specified location of the directory and/or filemark table becomes highly inefficient.
Alternatively, only a few large data files may be stored on a portion of the magnetic tape, such that locating a desired data file may be difficult.
One approach is to divide the magnetic tape data storage medium into regions. As one example, where the magnetic tape comprises one or more longitudinal tracks, a track, or group of parallel tracks, (also called a xe2x80x9cwrapxe2x80x9d if the tracks are arranged in a serpentine pattern), may be arranged into a small number of fixed regions having fixed boundaries. The directory references data sets at the boundary between regions, detecting the name of a file from information in a data set. Thus, if the filemark of a desired file is not at the data set at the boundary, the beginning location of the file is not known, requiring a time consuming operation to go backwards to a known data file and space to the filemark demarking the desired data file. In U.S. Pat. No. 5,572,378, all data files are required to begin at a boundary to reduce access time, but the data storage becomes inefficient in such an arrangement, wasting space between the end of a file and a boundary.
In accordance with the present invention, data storage systems, methods, and removable data storage cartridges provide data storage directories for data recorded on a magnetic tape data storage medium.
In one embodiment, the data storage system comprises data storage logic, which:
arranges a map of at least one longitudinal track of the magnetic tape data storage medium into a plurality of fixed regions;
establishes a window at each intersection between the fixed regions, the window extending into at least one of the regions;
determines whether at least one of the data demarcation signals is located in at least one of the windows;
if at least one of the data demarcation signals is located in one of the windows, selects one of the data demarcation signals for the window; and
identifies, in the directory, the selected data demarcation signal as within the window.
In a further embodiment, if no data demarcation signal is located in the window, the data storage logic identifies, in the directory, an absence of data demarcation signals within the window.
In a still further embodiment, the data demarcation signals comprise filemarks, and in another embodiment, wherein the data files are recorded in the form of at least one data record, the data records having secondary demarcation signals, the filemarks comprising primary demarcation signals. The data storage logic, additionally:
determines whether a filemark primary demarcation signal is located in a window; and
if no filemark primary demarcation signal is located in the window, determines whether a secondary demarcation signal is located in the window; if at least one of the secondary demarcation signals is located in one of the windows, selects one of the secondary demarcation signals for the window; and identifies, in the directory, the selected secondary demarcation signal as within the window.
In still another embodiment, where the magnetic tape data storage medium has longitudinal position signals encoded thereon, the data storage logic:
arranges a map of at least one longitudinal track of the magnetic tape data storage medium into a plurality of fixed regions;
establishes a window at each intersection between the fixed regions, the window extending into at least one of the regions;
determines whether at least one of the data demarcation signals is located in at least one of the windows;
if at least one of the data demarcation signals is located in one of the windows, selects one of the data demarcation signals for the window;
reads the encoded longitudinal position signals of the magnetic tape data storage medium at substantially the location of a data set of the selected data demarcation signal; and
identifies, in the directory, the selected data demarcation signal as within the window; and identifies, in the directory, the read location of the data set of the selected data demarcation signal.
For a fuller understanding of the present invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings.